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package com.oracle.truffle.api.dsl;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import com.oracle.truffle.api.Assumption;
import com.oracle.truffle.api.ContextLocal;
import com.oracle.truffle.api.ContextThreadLocal;
import com.oracle.truffle.api.TruffleLanguage.ContextPolicy;
import com.oracle.truffle.api.TruffleLanguage.LanguageReference;
import com.oracle.truffle.api.nodes.DirectCallNode;
import com.oracle.truffle.api.nodes.Node;

/**
 * Methods annotated with {@link Idempotent} must be methods that may cause side effects but will
 * always cause the same side effects for repeated invocations with the same parameters. From this
 * follows that an idempotent method must return the same result after it was executed once for each
 * consecutive execution given the same parameters.
 * <p>
 * Truffle DSL uses this property to execute {@link Specialization#guards() guards} where all bound
 * methods are idempotent once on the slow-path, during specialization, and never on the fast-path.
 * By default, all methods that do not bind dynamic parameters are interpreted as idempotent for
 * compatibility reasons. The DSL emits warnings whenever a method should be annotated either with
 * {@link Idempotent} or {@link NonIdempotent}. Note that guards that bind dynamic parameters will
 * never be idempotent in the fast-path, so the DSL will not require annotations for such guards.
 * <p>
 *
 * Examples for non-idempotent methods are:
 * <ul>
 * <li>{@link ContextThreadLocal#get()}
 * <li>{@link Assumption#isValid()}
 * <li>{@link Assumption#isValidAssumption(Assumption)}
 * <li>{@link Assumption#isValidAssumption(Assumption[])}
 * <li>{@link ContextLocal#get()} if the language uses a context policy {@link ContextPolicy#REUSE}
 * or {@link ContextPolicy#SHARED}
 * </ul>
 *
 * Examples for idempotent methods are:
 * <ul>
 * <li>{@link LanguageReference#get(Node)}
 * <li>{@link DirectCallNode#getCallTarget()}
 * <li>{@link ContextLocal#get()} if the language uses context policy
 * {@link ContextPolicy#EXCLUSIVE}.
 * </ul>
 * <p>
 * Note that usages may assume that the current language instance never changes for a given Node.
 * Therefore it is safe to assume language reference accesses idempotent.
 *
 * @see NonIdempotent
 * @since 23.0
 */
@Retention(RetentionPolicy.CLASS)
@Target({ElementType.METHOD})
public @interface Idempotent {

}
